The core of light-water-cooled nuclear reactors, and especially of nuclear reactors cooled by pressurized water, consists of assemblies of prismatic form which are juxtaposed and constituted in the form of bundles of fuel rods between which there are placed, in specified positions, guide tubes in which, when the reactor is in service, means for regulating the nuclear reaction are displaced. The fuel rods and the guide tubes are in vertical position in the core of the reactor.
Use is made of clusters comprising a plurality of parallel fuel rods which constitute the means for regulating the nuclear reaction, when they are introduced more or less deeply into the guide tubes.
Each of the control clusters used for operating the reactor is associated with a fuel assembly, the fuel rods of such cluster being disposed in an array corresponding to the guide tubes of the assembly. The guide clusters comprise a crosspiece or spider comprising a plurality of fins distributed angularly about the axis of the cluster and a central sleeve which permits the connection of the cluster to an operating rod or extension piece permitting the displacement of the cluster in the core. Each of the fins of the spider comprises at least one protective tube having a bore enabling attachment of a fuel rod on the spider. The fuel rods comprise an active part closed at its ends by plugs and extended on one side by a rod which comes into engagement in the bore of the protective tube, of longitudinal direction, to ensure the fixing of the fuel rod on the cluster.
The control clusters of the reactor have well defined functions, depending on the nature of the fuel rods constituting these clusters.
Certain clusters permit the regulation of the reactivity throughout the operation of the reactor, and especially the variation of its power. Other clusters contain a consumable poison and are introduced into the core exclusively during the first cycle of core operation in other clusters, the fuel rods simply constitute plugs intended to obtdurate the guide tubes of the assemblies in order to limit the throughput of coolant circulating in the assembly without coming into contact with the fuel rods; finally, other clusters permit the performance of a variation of the neutron spectrum of the core by modification of the moderator volume in the core.
It is clearly advantageous to construct the means for fixing the end part of the fuel rods on the spider in such a manner that these fuel rods remain demountable in case of excessive wear, of fracture or deterioration. This obviates the need to undertake complete replacement of the cluster. However, the elements for fixing the fuel rods on the spider must exhibit a high degree of safety in service and prevent any accidental separation the fuel rod and of the spider, for example under the effect of the vibrations in the core of the reactor.
Devices and processes have therefore been proposed for the demountable fixing of fuel rods on control clusters which theoretically diminish the risks of accidental separation of the fuel rod and the spider in service.
FR-A-2,439,457 discloses a process for positioning and withdrawal of fuel rods from a control cluster and the corresponding fixing device. The plug obturating the end part of the fuel rod to be assembled to the spider is extended by a deformable ferrule, the diametral expansion of which is carried out within the bore of the protective tube of the spider in order to ensure the fixing of the fuel rod.
The fuel rod may be demounted by exerting a force of longitudinal direction on the plug, in such a manner as to restrict the ferrule and to permit passage thereof into the bore of the protective tube.
In order to undertake the fixing and demounting of the fuel rod, the plug and the ferrule which extends it must be accessible by the end of the protective tube opposite to the end by which the fuel rod is engaged. The protective tubes of the spider must therefore pass through the fins from side to side; this presents disadvantages and corresponds to a particular type of spider.
Furthermore, in the course of the positioning or withdrawal of a fuel rod, forced support is taken on the bore of the corresponding protective tube. These operations may involve deformations of the bore, so that it is necessary to refinish or remachine the spider, or less frequently, in the case where a large number of replacements of fuel rods are undertaken. These refinishing or remachining operations are relatively complex, to the extent that the cluster has been activated under the effect of the bombardment of the neutrons while it has remained in the core of the nuclear reactor.
On the other hand, deformation of the metal constituting the ferrule at the end of the fuel rod results in the creation of a certain fragility, to the extent that the forces applied in the course of the deformation are not always easily controllable. This creation of fragility, may lead to an accidental separation of the fuel rod and the spider, under the effect of the vibrations to which the fuel rod is subjected in the reactor in service.
Moreover, the demounting of the fuel rod necessitates the exertion of a large force in the longitudinal direction on the plug in order to push the end part of the fuel rod through the bore. In the case of the appearance of jamming or of a hard point, relatively large jolts may take place, in the course of the demounting of the fuel rod, which run the risk of causing the appearance of fractures or of other deteriorations of the fuel rod which has been made particularly fragile by the irradiation.
FR-A-2,599,884 contains the proposal of another mode of fixing of the fuel rods, likewise requiring the presence of protective tubes, the bores of which pass through the fins of the spider from side to side.
The fixing rod extending the fuel rod is of a sufficient length for its end part to project in relation to the upper end of the protective tube, when the fuel rod is engaged, in the assembly position, into this protective tube. The projecting upper part of the rod comprises a thread on which there may be engaged an internally threaded hollow plug supported on the upper part of the protective tube and ensuring the fixing of the fuel rod, when it is screwed onto the threaded part of the rod. The fixing may be completed by making the assembly non-unscrewable under the effect of the vibrations, by virtue of deformable metallic elements integral with the rod, which elements may be turned down into cavities formed in the hollow plug.
This fixing device, while entirely satisfactory, is applicable only in the case of the spiders comprising protective tubes the bores of which pass from side to side through the fins in which they are placed.
A more common type of control clusters comprises a spider the protective tubes of which have blind bores threaded internally over a part of their length and through the opening of which there is introduced the assembly rod of a fuel rod which comprises a threaded part which is screwed into the internally threaded part of the protective tube. The assembly rod is made non-unscrewable under the effect of the vibrations, by introducing and welding a pin within a seating constructed at the end of screwing of the assembly rod of the fuel rod, by placing into alignment of an opening passing through the protective tube and a blind hole provided in a large-diameter part of the assembly rod constituting a centering ring.
This mode of locking the fuel rod against rotation is difficult and tricky to implement; this complicates the operations of replacement of a fuel rod on a control cluster.